Lens
A lens is an optical device through which light is focused in order to form an image inside of a camera either on film or on a digital sensor. Anatomy of a Lens Elements An element is an individual glass structure in a lens that is not permanently, directly fixed to any other element. An element may be an individual piece of glass (itself a lens), or it may consist of two or more pieces of glass which are cemented together. (Older lenses used Canada balsam, made from pine pitch, for cement, while newer lenses use higher-tech UV-cured cements.) Barrel The barrel is the tube-shaped outer shell that contains the lens elements. Aperture An aperture is an opening in a lens that limits the amount of light passing through it to the camera body. The term aperture is also often used to describe the diameter of that opening. It is usually adjustable, most often by turning an "aperture ring" on the outside of the lens body. Mount A lens mount is the part of an interchangeable system lens that connects to the camera body. Geometry of a Lens Optical Axis The optical axis is an imaginary straight line which passes through the centers of curvature of the lens elements and meets the focal plane at a 90-degree angle. Focal Plane The focal plane is the flat plane onto which a lens focuses its image. The focal plane sits at a 90-degree angle to the optical axis. The film or digital sensor should sit on the focal plane. Focal Length The focal length of a lens is defined as the distance in (usually in mm.) from the optical center of the lens to the focal plane, which is located on the sensor or film if the image is "in focus". Fixed and Interchangeable Lenses Fixed Lenses A fixed lens is simply a lens that is permanently fastened to its camera as opposed to a system camera that allows different lenses to be used on the same camera easily. Fixed lenses are commonly found cameras aimed at consumers, from old box, TLR (the Mamiya C systems being the exception) and folding cameras, through the consumer range finders of the '70s and through to today's point and shoot film and digicams. There are certain advantages to having a fixed lens on your camera: # As no mechanism for changing lenses needs to be built into the camera design it can help keep the camera smaller and lighter. # In digital cameras a fixed lens means that there is less chance of introducing dust to the sensor surface. # Fixed lenses are designed for a specific camera model and so fewer compromises have to be made in the lens design. # Cost - if your camera comes with a fixed lens you don't have to worry amount buying a lot of additional glass to build a system. # Portability - a fixed lens should be enough for most situations you encounter so you have less accessories to carry and you will waste less time changing lenses. Interchangeable Lenses Interchangeable lenses are more commonly found on cameras aimed at professionals and enthusiasts including large format, SLR (medium format and 35mm) and high-end rangefinder cameras. The advantages to interchangeable lenses include: # A larger range of focal lengths and specialties (shift, macro, etc.) are available than you are likely to find on any fixed lens camera. # Each lens can be designed for a specific kind(s) of working situations and specialties without the compromises a generalist fixed lens has to be designed for. # Longevity - you can upgrade your camera body without losing any investment you have made in additional lenses if your new camera choice is in the same family as your old camera. A given interchangeable lens body can accept one type of lenses. There are cases of compatibility, when different bodies share the same lens mount. Adapters can exist to put a lens designed with one type of lens mount on a body designed for another. Auxiliary lenses - if your camera has a fixed lens there are accessories available that allow you to enhance your fixed lenses range. These included close-up lenses that allow your camera to focus closer than it naturally can. They also include wide-angle attachments that allow your fixed lens to capture more of a scene than it otherwise could. They also include popular telephoto attachments that allow your fixed lens to reach further than it otherwise could. Telephoto attachments include extreme digiscoping lenses. Digiscoping is the practice of mounting a digicam on a spotting scope of telescope to create extreme focal lengths. As with all photographic equipment, auxiliary lenses range in quality from the truly dreadful to the professional. Bear in mind that any auxiliary lens that you attach to your fixed lens is adding more glass between the subject and the film. As such it is bound to affect image quality and the amount of light passing through to the film plane. Cheap auxiliary lenses add horrible distortion and purple fringing to your shots. Auxiliary lenses are a compromise solution to extending the range of a fixed lens that can provide good results but there appear to be no bargains in this niche market place and you will get what you pay for. Alphabetical list of lens mounts For a list of lens mounts, see Lens mount. Zoom and Prime Lenses Prime The term prime refers to a lens with a single focal length. Typically, prime lenses have larger maximum apertures, so they are able to let in more light wide open than similar zoom lenses. This makes prime lenses more suitable to low-light photography. Zoom A zoom lens is a compound lens with a variable effective focal length. While (contrary to a popular misconception) the perspective does not change, shifting the focal length of a zoom lens does allow the photographer to modify the crop of a photo without moving. Zoom lenses are bulkier than fixed lenses, and they introduce an extra adjustment you must make before taking the picture. The vast majority of digital cameras come equipped with Zoom lenses. The zoom ratio is the ratio between the shortest focal length and the longest focal length of a given lens. The majority of modern zoom lenses are about 1:3, meaning that their longest focal length is 3 times the shortest. For example, there are many 35-105 lenses available. As the ratio gets bigger, the lens becomes much harder to manufacture, and more expensive. Some modern digital cameras have zoom ratios of 1:10, or even 1:12. It may be that such a camera could lessen the need for interchangeable lenses, and perhaps these will become more of the norm. Currently, they represent the leading edge of consumer optical technology. Frequently, lenses for digital cameras are labelled with the focal length they would have if they were 35mm cameras. This gives a way of comparing zoom ratios between film and digital cameras. In any case, divide the larger number by the smaller. If the result is less than 3, then it's unimpressive. If it's about 3, then it's a normal, conservative design. If it's much greater than 4, some testing might be in order. Classifications Normal A normal lens has an angle of view that approximates how the human eye sees a scene. A lens is considered normal when its focal length is approximately equal to the diagonal of the film format. Lenses shorter than normal are called wide-angle, while those longer are called telephoto. In 35mm photography, 50mm is considered to be the normal focal length, even though the actual diagonal of the frame (24mm x 36mm) is 43mm. For medium format photography (frame size 2-1/4" square, or 6x6cm), normal is generally 80mm. Wide-angle A wide-angle lens is a lens with a focal length shorter than normal. A wide-angle lens, as opposed to a fisheye, is normally well-corrected for geometrical distortion, i.e., straight lines appear straight. However, as the angle of view is wider than what is seen by the human eye, they can create an impression of distortion; this perceived distortion increases as the focal length of the lens decreases. A fisheye lens (see below) is an extreme example of a wide-angle lens. Telephoto A telephoto lens is any lens with a focal length longer than normal. A telephoto lens brings far subjects closer, like a telescope, hence the name. They therefore have a smaller angle of view than a normal or wide-angle lens. The longer the lens, the more likely that camera shake will blur the image; for this reason, longer telephoto lenses are usually used with a tripod to steady the camera. Fisheye Fisheye lenses have the widest field of view of any lens group. The geometrical distortion of the image is purposefully maintained high, and straight lines appear curved if they are near the edge of the image. This creates distortion of the resulting image in a dramatic way. Fisheye lens fall into two categories: * Circular fisheyes: have a 180 degree field of view when measured along the smallest dimension of the image, resulting in a circular image with black corners. In 35mm format, they usually have a focal length around 8mm. * Full frame fisheyes: have a 180 degree field of view when measured along the diagonal, so the image extends on the full film plane. In 35mm format, they usually have a focal length around 16mm. In general, fisheye lenses are expensive and little used in everyday photography. Fisheye adaptors: auxiliary lenses are available that simulate a fisheye field of view. This is cheap way to play with the fisheye effect without investing in a dedicated fisheye lens, although, as with most auxiliary lenses, the quality of your images will not be the same as those taken using a 'real' fisheye. Lens Speed Lenses that let in a lot of light are called 'fast' lenses. This quality is indicated by a number which is called the maximum aperture or maximum f stop. The smaller the number, the faster the lens. A fast lens will typically have a maximum aperture of f2.0 or less. The aperture number is the focal length divided by the diameter of the lens. As lenses get faster, they become more difficult to make, larger, and more costly. Fast lenses are important if you wanted to take photos in dim light without flash, and without a tripod. Lens Coating Modern lenses are coated with a very thin layer of antireflective material (like magnesium fluoride or calcium fluoride). This lens coating is applied to the each element of a lens which has a surface exposed to air. The only purpose of coating lenses is to reduce lens flare by eliminating reflection off the surface of the glass; this has the effect of increasing contrast and giving images more "punch". Lens coating thicknesses are typically measured in units of wavelengths of light, in nanometers (nm). Lens coatings have nothing to do with color correction of lenses, as is widely thought. Multicoating refers to the application of more than one layer of coating on a lens. All modern lenses are coated. Coating gives lenses their colored reflective look. Since lens coatings are relatively fragile, one should always be careful when cleaning coated lens surfaces (like the front of the lens), so as not to scratch or smudge them. * For an explanation of how lens coatings work, check here. Depth of field The depth of field is a way of describing how much of your image is in focus. When the camera is focused at a certain point, it will remain in focus for objects slightly in front of that point, as well as slightly behind. The distance between the closest object that is in focus, and the most distant one, is the depth of field. The depth of field is dependent purely on the geometry of the lens, and cannot be changed by the manufacturer. Generally, the shorter the focal length of the lens, the greater its depth of field. Zoom lenses have more depth of field when set to their shortest focal length, than when set to the longest. Since all small digital cameras have lenses with very short focal lengths, they tend to have very large depth of field. This has many benefits, and generally makes the job of the autofocus mechanism much easier. On the other hand, certain aesthetic effects become more difficult when the lens has too much depth of field, for example having a sharp subject emphasized over a blurry background. Finally, the faster the lens, the lower the depth of field. This means that while using a very fast lens will allow you to photograph in dim light, it will be very difficult to adjust the focus when you do this. Moral: if you think you want a very fast lens, you will pay for it in cost, weight, bulk, and poor depth of field. But of course you will be able to take pictures in dim light, and to detach your subject from the background in more situations. * Depth of field explained by Ching-Kuang Shene Angle of view In general, the shorter the lens, the wider it's angle of view. It's much easier to get very wide angle lenses for film cameras than for digital, unless you are talking about very expensive digital cameras. Many digital cameras suffer from a lack of wide angle ability, and if that's important to you, that will affect your choice. If you must have very wide angles, you will need to get a camera with interchangeable lenses, whether film or digital. Some lower cost digital cameras can be fitted with add-on lenses that increase their angle of view. In 35mm cameras, a 50mm lens is said to give an angle of view similar to the human eye, though many folks dispute this. nevertheless, this has come to be called a standard lens, for 35mm cameras. Distortion This is when the lens represents straight lines as bent. This can be often seen in zoom lenses at both ends of the zoom range, straight lines at the edge of the frame will appear slightly curved. There seems to have arisen a kind of accepted dogma that this is a bad thing, though nobody seems to explain why. In fact it depends on the type of pictures you will take. It will usually be bad with architectural pictures, with pictures full of geometrical shapes, and for the reproduction of flat objects such as paintings. It will usually not matter for portraits and for daily shooting. If you think it matters for you, then check carefully for that behavior in any lens you are choosing. Once again, it seems to be difficult for lens manufacturers to achieve very low distortion in conjunction with all the other good features they want their lenses to have. The lenses on which distortion seems the most difficult to correct are the wide angle lenses. Barrel Distortion When straight lines bow out towards the edge of the frame (like the profile of a barrel) it is known as barrel distortion. This is typically found to some extent at the wide end of many zoom lenses. Pin cushion Distortion When straight lines bow in from the frame edge it is known as pin cushioning. This is typically found at the long end of zoom lenses. Links * Explanation with photos, Courtesy of toxpose.com * In Praise of the Standard Prime Lens * Lensbabies * Simple lens tests are found at this Kiev 60 page